With decades of technical advancements and breakthroughs in the areas of semiconductor based solid-state light emitting devices, Edison's incandescent light bulbs, which typically have 30% or less light efficiency, will soon be replaced with energy-efficient light-emitting diodes (“LEDs”). A conventional LED is small and energy efficient with good lifetime. Various commercial applications of LEDs, such as homes, buildings, traffic lights, and electronic billboards, have already placed in service.
An LED is a semiconductor diode with a biased p-n junction capable of emitting narrow-spectrum light or electroluminescence. Color of emitted light typically depends on the composition of material used in the device. Color variations for visible light are usually defined by electromagnetic radiation or optical wavelengths from approximately 400 nm (nanometer) to 700 nm.
To generate white light, a conventional approach is to combine multiple emission wavelengths of LED sources (e.g., red, green, and blue LEDs) to produce desirable white light with various correlated color temperature (CCT). Alternatively, LEDs may be combined and/or added with luminescent material such as phosphors to convert at least a portion of LED emitted light to longer wavelength emissions to achieve a combined emission spectrum with white light of various CCT.
The quality of a white light source on color appearance of objects is usually measured by the Color Rendering Index (“CRI”), wherein the highest CRI rating is 100 when the alternative light source closely mimics the radiation spectral distribution of incandescent object at the same color temperature. Typical cool white fluorescent lamps, for example, have a CRI of 62. On the other hand, lamps having multiple component rare-earth phosphors can be constructed with CRI of 80 or better.
To enhance color appearance, a conventional approach is to mix blue LED(s), red LED(s) and some luminescent material that responds to blue emission in a package to generate warm or neutral white light. A problem associated with placing blue LED(s) and red (or orange) LED(s) in a same package is the color shift which is due to different temperature-dependent radiant (or luminous) flux drop as temperature drops. A factor that causes color shift is that the radiant (or luminous) flux drop between blue LED and red LED is different as operating temperature rises. For example, conventional indium gallium nitride (“InGaN”) based blue LED chip and aluminum gallium indium phosphide (“AlInGaP”) based red LED chip typically have different rate for flux drop in response to temperature change.